The field of the invention is display devices, such as cathode ray tubes, which are used within strong magnetic fields such as those produced by nuclear magnetic resonance (NMR) scanners.
NMR scanners are finding wide application in hospitals and clinics where they are employed to produce images of patients. NMR scanners employ very strong magnetic fields (0.5 to 1.5 Tesla) and are usually enclosed in a specially constructed room which isolates the scanner from radio frequency noise. The operator's console is positioned near the scanner where the patient can be observed and it is well within the strong magnetic field. The operator's console employs one or more displays which are used to display data and to produce images of the patient. Such displays usually employ cathode ray tubes (CRT) that project a beam of electrons onto a screen.
Severe distortion results when CRT displays are used in strong magnetic fields. An electron moving through a magnetic field is subject to forces that act to displace it from its intended path. This results in the electron beam not striking the phosphor screen in the proper location with a consequent distortion of the CRT image. When located near the NMR scanner, a CRT image is distorted to the point where it is virtually useless. While this distortion can be reduced by moving the display further away from the scanner, this is often not possible due to space limitations. The problem has been exacerbated by the use of large deflection CRT geometries which have longer beam paths and larger deflection angles.
The method presently used to reduce the effects of a magnetic field on the electron beam is to shield the CRT display with a ferromagnetic material. A housing of steel and mu metal is constructed arond the CRT and an opening is provided through which the screen can be viewed. While shielding is quite effective, the magnetic field still enters through the screen opening to produce distortion. This distortion can be further reduced by extending the shield forward from the CRT screen face, but such structures limit the angle at which the CRT display can be viewed.